1. Field of the Invention
The present invention relates to a magnetic disk unit for storing data, more particularly, it relates to the magnetic disk unit which reduces the difference of level of respective regenerative signal read out from a plurality of magnetic heads.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a gain control system of a conventional magnetic disk unit disclosed in Japanese Patent Laid-Open No. 61-199274(1986), in which numeral 1 denotes a magnetic recording medium and numeral 2 denotes a magnetic head which records and reproduces the data relative to the magnetic recording medium 1. Though the regenerative signal from each magnetic head 2 is inputted to an amplifying circuit 3 connected separately thereto, among these regenerative signals only the one outputted from the magnetic head 2 which is selected by a head selector circuit 4 is amplified at a constant gain in the amplifying circuit 3 and inputted to an automatic gain control amplifying circuit 5.
The automatic gain control amplifying circuit 5 is designed to increase or decrease the gain in response to the gain control signal which is an output from an AGC signal generating circuit 6 or from a fixed gain signal generating circuit 8 inputted selectively through a signal switching circuit 7 to amplify the signal inputted from the amplifying circuit 3 to be outputted. The signal switching circuit 7 is designed to be switched to the fixed gain signal generating circuit 8 while the magnetic head 2 is recording the data and to the AGC signal generating circuit 6 while reproducing the data.
The AGC signal generating circuit 6 is constituted to output the gain control signal in response to the regenerative signal level inputted from the automatic gain control amplifying circuit 5.
In addition, the fixed gain signal generating circuit 8 is designed to output the constant gain control signal regardless of the regenerative signal level.
In such conventional magnetic disk unit, among the regenerative signals from a plurality of magnetic heads 2 located adjacent to the magnetic recording medium 1, only the one from the magnetic head 2 selected by the head selector circuit 4 is, after being amplified to a constant level in the amplifying circuit 3, amplified in the automatic gain control amplifying circuit 5 in response to the gain control signal inputted from the signal switching circuit 7 as its gain being adjusted by increasing or decreasing, and inputted to the AGC signal generating circuit 6.
The signal switching circuit 7 inputs the gain control signal from the fixed gain signal generating circuit 8 to the automatic gain control amplifying circuit 5 and keeping a mu-factor at constant while the magnetic head 2 is recording the data, and at the early stage of inputting the regenerative signal to the AGC signal generating circuit 6, maintaining the gain of the automatic gain control amplifying circuit 5 below maximum, and simultaneously with the beginning of reproduction, switching to the signal from the AGC signal generating circuit 6 which generates the gain control signal in response to the regenerative signal level.
Now, in the conventional magnetic disk unit described hereinabove, when the magnetic head is in the recording state, the automatic gain control amplifying circuit 5 amplifies and controls in response to the gain control signal from the fixed gain signal generating circuit 8, so that the gain is maintained at a prescribed value, but for example, when the magnetic head 2 is switched over to the reproducing state without becoming the recording state or is moved diametrically on the magnetic disk as keeping the reproducing state, in the beginning, due to the difference of regenerative data level between the magnetic heads 2, the gain immediately after switching becomes too large or too small in response to the occasional reproducing data level, thus a voltage amplitude of the regenerative signal becomes unstable in the beginning of read operation results in an inaccurate data propagation.